The present invention relates generally to the field of electric motors and more particularly to apparatus for controlling the speed of rotation of such motors.
As is known, electric motors have a wide variety of applications. In many applications it is desirable that the speed of rotation of the motor be controlled. One apparatus for controlling motor speed detects actual motor speed with a tachometer and generates a control signal representing the difference between the actual and desired motor speeds. The motor is nominally powered by full-wave-rectified AC voltage. The control signal fires a silicon-controlled-rectifier (SCR) in the motor's power line when motor speed is slower than that desired, allowing the rectified voltage to be applied to the motor and thus increasing motor speed. The control signal shuts the SCR off when the speed of the motor exceeds the selected speed, thereby decoupling power from the motor and thus slowing down the motor. While such apparatus performs well in some applications, the apparatus produces large current variations in the motor because of residual ripple in the rectified AC supply voltage, thus increasing the response time of the control apparatus. Also, the motor is pulsed with power at roughly the frequency of the fully rectified AC voltage (120 Hz for 60 Hz AC). Such frequency may be close to the mechanical resonant frequency of an object mechanically coupled to, and driven by the motor resulting in vibration in the object with concomitant hum as power is coupled to and decoupled from the motor.
Another system uses pulse width modulation of a high frequency pulse train to control the speed of a motor by regulating the average, or DC, current level applied to the motor. More particularly, the actual speed of the motor is detected by a tachometer and a control signal is generated in accordance with the difference between the actual speed of the motor and a selected speed. A pulse width generator produces a pulse train having a nominal frequency and duty cycle (pulse width). The duty cycle of the pulse train is modulated in accordance with the control signal. The modulated pulse train is coupled to a switch, such as a transistor, in series with the DC motor voltage supply. The switch closes during each pulse and remains open between pulses. Thus, when the motor is rotating faster than a selected speed the modulator reduces the duty cycle of the pulse train, resulting in decreased power being applied to the motor thereby reducing the motor speed. If the motor turns too slowly, the duty cycle of the pulse train is increased by the modulator, thereby permitting greater power to be applied to the motor to increase motor speed.
While such a system performs well in most applications, it relys on a stable and precise DC motor supply voltage and current. Further, such system may not respond quickly enough in some applications to variations in the DC motor voltage and current. Thus, if the DC motor supply voltage is produced by rectifying an AC voltage supply, complex filtering must be used to substantially eliminate line ripple produced on the DC voltage to ensure a precise DC motor voltage and current level. Additionally, in some applications the motor is used to drive a high inertial load. Because of the inertia of the load, when the control signal directs the motor to reduce speed and stop from a previously selected speed, the load applies mechanical force to the motor. The motor is thus forced to continue to turn, such residual rotation being undesirable.